A storage controller is a physical processing device that is used to store and retrieve data on behalf of one or more hosts. A network storage controller can be configured (e.g., by hardwiring, software, firmware, or any combination thereof) to operate as a storage server that serves one or more clients on a network, to store and manage data in a set of mass storage devices, such as magnetic or optical storage-based disks or tapes. Some storage servers are designed to service file-level requests from hosts, as is commonly the case with file servers used in a network attached storage (NAS) environment. Other storage servers are designed to service block-level requests from hosts, as with storage servers used in a storage area network (SAN) environment. Still other storage servers are capable of servicing both file-level requests and block-level requests, as is the case with certain storage servers made by NetApp®, Inc. of Sunnyvale, Calif., employing the Data ONTAP® 7G or GX storage operating systems.
In at least one implementation, a storage server makes data available to a client (host) system by presenting or exporting to clients one or more logical containers of data, such as volumes. A “volume” is an abstraction of physical storage, combining one or more physical mass storage devices (e.g., disks) or parts thereof into a single logical storage object, and which is managed as a single administrative unit, such as a single file system. A “file system” is a structured (e.g., hierarchical) set of stored logical containers of data (e.g., volumes, logical unit numbers (LUNs), directories, files). Note that a file system does not have to include storage that is based on “files” per se.
From the perspective of a client system, each volume can appear to be a single disk drive. However, each volume can represent the storage space in a single physical storage device, a redundant array of independent disks (RAID) or a RAID group, an aggregate of some or all of the storage space in multiple physical storage devices, or any other suitable set of storage space. An “aggregate” is a logical aggregation of physical storage; i.e., a logical container for a pool of storage, combining one or more physical mass storage devices (e.g., disks) or parts thereof into a single logical storage object, which contains or provides storage for one or more other logical data sets at a higher level of abstraction (e.g., volumes).
A network storage system can have a very simple architecture; for example, an individual storage server can provide one or more clients on a network with access to data stored in a mass storage subsystem. As another example, two or more individual storage servers may be connected and configured as failover partners, to increase reliability, such as in a storage failover (SFO) or controller failover (CFO) configuration. Recently, however, with storage capacity demands increasing rapidly in almost every business sector, there has been a trend towards the use of clustered network storage systems, to improve scalability.
In a clustered storage system, two or more storage server “nodes” are connected in a distributed architecture. The nodes are generally implemented by two or more storage controllers. Each storage server “node” is in fact a storage server, although it is implemented with a distributed architecture. For example, a storage server node can be designed to include a network module (“N-module”) to provide network connectivity and a separate data module (e.g., “D-module”) to provide data storage/access functionality, where the N-module and D-module communicate with each other over some type of physical interconnect. Two or more such storage server nodes are typically connected to form a storage “cluster”, where each of the N-modules in the cluster can communicate with each of the D-modules in the cluster.
A clustered architecture allows convenient scaling through the addition of more N-modules and D-modules, all capable of communicating with each other. Further, a storage cluster may present a single system image of stored data to clients and administrators, such that the actual location of data can be made transparent to clients and administrators. An example of a storage controller that is designed for use in a clustered system such as this is a NetApp FAS-series Filer which employs NetApp's Data ONTAP® GX storage operating system.
Both clustered and non-clustered network storage systems have advantages. However, in storage server products available today these two types of systems are mutually exclusive. That is, a storage server is generally designed to operate as part of a clustered storage system or as a non-clustered storage server, but not both. This places limitations upon storage system growth and performance over time and inhibits the most efficient use of storage resources.
For example, a company may use a non-clustered storage system with satisfactory results for a long time, but may conclude at some point in time (e.g., in anticipation of significant corporate growth) that it would be desirable to have a clustered storage system. With conventional technology, the company would have to replace its entire storage system with clustered-enabled devices, even though the existing non-clustered storage system it is using may be fully functional and adequate for many purposes. The company is therefore faced with either continuing to use the existing storage system which becomes less and less optimal over time, potentially adversely affecting business processes, or purchasing an expensive new system and giving up the use of the (also expensive) fully functional existing system. Either choice is likely to prove costly.